1. Field of the Invention
This invention relates to direction finding antenna systems and, more specifically, to a multi-octave, instantaneous, full azimuthal field of view (FOV) direction finding (DF) antenna.
2. Brief Description of the Prior Art
Electronic information gathering systems require accurate, multi-octave DF performance including low angle of arrival (AOA) error and large FOV, optimally a full 360 degree azimuthal FOV. Most prior art DF systems are normally grouped into two operational categories, these being (1) amplitude only DF and (2) phase DF.
Amplitude only DF is limited in accuracy, and typically can only be used for quadrant information in the azimuthal plane unless a gimbal system is used. In order to achieve very accurate DF, a phase DF approach must be used.
In the prior art, two basic approaches have been used to realize the above mentioned DF techniques, these being (1) fixed circular arrays of individual antennas and (2) a mechanically gimballed antenna or array of antennas.
The first approach has been to use a circular array of individual antennas, oriented radially outward in sufficient numbers to provide the required 360 degree FOV coverage. The number of antennas placed in the array is dependant upon the beamwidth of the individual antennas in the array. This approach suffers from several disadvantages, these being size and frequency band limitations. To achieve high gain, multi-octave performance, antennas (normally termed frequency independent antennas) become electrically large. An above mentioned circular array of such antennas would in turn require a large amount of volume. Due to the large electrical size of the frequency independent antennas, the associated phase centers of the antennas are spaced electrically distant. In a circular array configuration, as the operating frequency of the array increases, the electrical separation between these phase centers becomes greater than one half wavelength, this in turn generating grating lobes in the far field radiation patterns of the circular array. These grating lobes distort the measured phase response of the array, thereby greatly increasing the AOA errors of the DF system. These AOA errors limit the upper operating frequency of the DF circular array. Accordingly, the frequency bandwidth of the prior art generally does not exceed a single octave.
Another deficiency encountered in the antenna array approach is the destructive interaction between the individual antennas. Mutual coupling between the individual antenna elements, which conceptually operate independently, degrades far field radiation pattern performance. This interaction corrupts the desired phase response of the individual antennas, consequently reducing the DF performance of the entire system.
The second technique uses a single antenna or an array of antennas placed upon a mechanical gimbal. The gimbal is rotated to provide the required FOV. This technique requires very accurate computational correlation between the physical location of the gimbal and the received signal of the antenna system. In order to achieve this accurate correlation with sufficient reliability, the complexity and cost of the gimbal/DF system must be greatly increased. An additional disadvantage of a gimballed system is that it can not provide an instantaneous 360 degree FOV. For a gimbal DF system to achieve performance approaching that of an instantaneous system, the gimbal must have extremely fast scanning speeds, further increasing the complexity, cost and unreliability of the system.
It is therefore readily apparent that the deficiencies in the prior art do not allow for the construction of multi-octave, low profile and reliable direction finding antenna systems that provide the instantaneous full 360 degree azimuthal coverage required in electronic intelligence gathering applications.